The invention relates to an apparatus for analogue information transfer which comprises a transmit circuit and a receive circuit. The transmit circuit communicates with the receive circuit through a transmission means, said transmit circuit generating, from a first analogue signal, a first pulse signal which is transferred to the receive circuit via the transmission means. The receive circuit generates a second analogue signal from a second pulse signal. The apparatus finds application in an extremely noisy environment.
Today digital signals are used in an ever increasing number of electronic processes. A digital signal consists of a row of pulses where each pulse consists of a rising flank and a descending flank, and where precisely the spacing between the flanks is decisive for the information in the signal. The variation in the flanks thus means that pulses with an individual mutual spacing and width are obtained, and in some signals precisely these factors are essential to the information in the signal. For the right information to be demodulated from these signals, it is also important that the flanks are uniform so that their occurrence is well-defined.
Conversion of analogue signals into digital signals and subsequent transmission of the digital signals involve the risk that noise pulses occur in the signal and that the signal is distorted. This may happen e.g. when the digital signal is communicated between two media, or when the signals move around in an electronic circuit because of interference, time delays or unilinearities in the electronic components or in the communications line.
Likewise, capacity between the conductor path or in the communications line will attenuate the high frequencies of the pulses more than the low ones and thereby round all the pulses. An analogue value based on a distorted digital signal may be vitiated by serious errors.
As a consequence of the noise pulses it may be difficult to distinguish between one of the original pulses and a noise pulse when the digital signals are demodulated, and at worst this may mean that the demodulated signal is misinterpreted. This misinterpretation may also occur because of distortion of the signal.
One way of avoiding the noise pulses is to filter the signal, but particularly in case of fast signals this filtering causes additional distortion of the signal since the filter cannot distinguish between a pulse and a noise pulse. When higher order filters are used, the filtering results in a different type of distortion of the signal as the signal history repeats itself in the later signal course. These distortions are very unfortunate in the digital signals where the integrity is of great importance. Integrity in a signal is taken to mean a signal where it is important to maintain a great accuracy with respect to the pulse sizes and the mutual positions of the pulses. These factors are essential to achieve the correct data in connection with decoding of the signal.
U.S. Pat. No. 4,027,152 discloses an apparatus for transmitting binary coded information over a fiber-optic link, which provides a link monitor to indicate whether the fiber optic link is intact and operating. The binary coded information is translated into a pulse coded signal which provides a positive pulse for positive going transition in the binary signal and a negative pulse for negative going transition in the binary signal. In addition a refresh pulse of the same polarity as the preceding pulse is provided whenever there has been no pulse for a predetermined amount of time.
The use of positive and negative pulses and a neutral level there between means that a three level analogue signal has to be transmitted over the optic fiber. Use of thee levels means that the sensitivity to noise is high because suppressing of noise between the levels is very difficult. The apparatus is not effective operating in an electric environment with a high level of noise.
The object of the invention is to achieve a precise transfer of an analogue signal through a transmission means in a noisy environment.
This may be achieved in that the first pulse signal consists of a series of coded signals, wherein a first code marks a positive flank, while a second code marks a negative flank. The receive circuit comprises a shape generator which generates a shape signal with uniform and well-defined flanks from the second pulse signal, wherein the spacing between the flanks approximately corresponds to the spacing between a first code and a second code in the first pulse signal. Rising and falling edges of the binary information are aligned with clock markings, where a refresh pulse identical with the preceding information pulse is generated, whenever the digital information does not have an edge at the respective clock marking. The receive circuit moreover contains a frequency generator which generates a clock signal on the basis of the coded pulses of the second pulse signal. The receive circuit also contains a signal generator which generates a third pulse signal from the clock signal and the shape signal, said third pulse signal generating the second analogue signal after D/A demodulation.
This provides a very precise transfer of an analogue signal through a transmission means, where the transferred analogue signal is unique in having maintained a very precise phase, and the transfer in general is unique in having a good linearity and a good noise suppression. The use of coding suppresses all other signals which do not live up to the coding criterion. Further, the coding form involves a power saving in the signal transfer, including the transmission means, which may be an advantage e.g. if the transmission means is an optocoupler.
In a particular embodiment of the apparatus, the first pulse signal transmits a code pulse at fixed intervals. It is hereby possible to transfer a clock signal and code information to the receive circuit, where the clock may be contained in the code.
In a particular embodiment of the apparatus, the first pulse signal contains a code pulse for a transmit clock signal. It is hereby possible to transfer code information to the receive circuit, whereby the clock signal may be restored precisely.
The first pulse signal may contain a first code for marking a positive flank and a second code for a negative flank, said first and second codes being transmitted at fixed intervals in dependence on an original signal. It is hereby possible to transfer a square-wave signal.
The original signal may be converted by a code generator into the first pulse signal, said code generator supplying a clock and performing coding in dependence on the clock. It is hereby possible to transfer a clock signal and a code signal to the receive circuit, where the clock may be contained in the code.
Advantageously, the original signal is generated by an analogue to digital modulator. This provides a digital signal which is less sensitive to noise than the analogue signal.
The original signal may be generated by a sigma delta modulator. This provides a digital signal which is less sensitive to noise than the analogue signal, and which can simultaneously operate at higher frequencies.
In a particular embodiment of the apparatus, the receive circuit recognizes coded pulses, and the original signal is restored on the basis of the codes. This provides a receive circuit which is capable of restoring a square-wave signal.
The receive circuit is capable of restoring the original signal by generating a positive state of the signal on the basis of the first code, while the second code generates the negative state of the signal. This provides recognition of the original signal on the basis of well-defined codes.
The apparatus is capable of restoring the clock from the coded signal in the receive circuit, said restored clock being used for determining the flanks of the signal. This provides a receive circuit which is capable of generating a clock signal from a coded signal, and which is capable of restoring the original signal from the clock signal and the code signal.
The apparatus is capable of generating a clock on the basis of the received signal in the receive circuit. This provides a receive circuit which is capable of generating a clock signal from a received signal.
In a particular embodiment of the apparatus, the receive circuit contains low-pass (LP) filtering of the second pulse signal. This provides increased noise immunity of a pulse signal.
Advantageously, the apparatus may incorporate a frequency generator with a phase-locked loop. This provides a clock signal, which is synchronous with the received signal.
The frequency generator can search for the clock within a predetermined frequency interval. This provides a clock signal which is synchronous with the received signal within a given frequency range.
The apparatus may contain the shape generator built as an analogue switching element, such as e.g. a Smith trigger. This provides a simple shape generator, which takes up very little space.
The signal generator may be composed of a D flip-flop. This provides a simple signal generator, which takes up very little space.
Advantageously, the second analogue signal is generated in the receive circuit by means of a digital to analogue demodulator. The second analogue signal is hereby restored in a simple manner.
The transmission means may be a transformer, an optocoupler, including a light guide connection or other galvanic separation face. This provides a good separation face between transmit circuit and receive circuit.